Personally I think that you shouldn't, in any case, get both the girls together. If you have activated Isabelle's route, then you cannot go to Kate's without breaking your relationship with Isabelle. Same the other way around. And I mean you can't in ANY case.
Example: Even if you broke Kate after Isabelle's party and Lindsey's fall, you should not be able to "fix" Kate in Isabelle route. Isabelle is fair, but not lacking in spite. If you want to go and fix Kate after everything that happened, you will have to break up with Isabelle. Maybe in the future you can fix your relationship with Belle, but only as friends.
I say all of this because I believe this game should not feature an harem in any case. We've already seen that going out with multiple girls at the same time have consecuences (speaking about how important Lindsey is for you with Isabelle after the season change gives Lindsey points, but you lose Isabelle points) and opening some doors close other doors (you can't take Lindsey's route and Maxine's at the same time). You can, of course, have multiple girls as long as they don't step on each others' paths (like Flora and Lindsey, for now).
The game is called Another chance. Another chance in life. A life in which you gain something after giving up on something else. After all, that's how life works.
